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Date: Thu, 12 May 2011 06:55:58 -0400
From: John Baldwin <jhb@FreeBSD.org>
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To: Stanislav Sedov <stas@FreeBSD.org>, neel@FreeBSD.org
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Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org,
	src-committers@FreeBSD.org, Jung-uk Kim <jkim@FreeBSD.org>,
	Andriy Gapon <avg@FreeBSD.org>
Subject: Re: svn commit: r221703 - in head/sys: amd64/include i386/include
 x86/isa x86/x86
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On 5/12/11 6:55 AM, Stanislav Sedov wrote:
> On Thu, 12 May 2011 13:43:58 +0300
> Andriy Gapon<avg@FreeBSD.org>  mentioned:
>
>>
>> Theory:
>> - smp_rv_waiters[2] becomes equal to smp_rv_ncpus
>> - [at least] one slave CPU is still in the last call to cpu_spinwait() in
>> smp_rendezvous_action()
>> - master CPU notices that the condition is true, exits smp_rendezvous_cpus() and
>> calls it again
>> - the slave CPU is still in spinwait
>> - the master CPU resets smp_rv_waiters[2] to zero
>> - the slave CPU exits spinwait, see smp_rv_waiters[2] with zero value
>> - endless loop
>>
>
> That might explain it.
> Do you have a patch for me to try?
>
> Thanks!
>

The NetApp folks working on BHyVe also ran into this.  They have a fix 
that I think sounds reasonable which is to add a generation count to the 
smp rendezvous "structure" and have waiting CPUs stop waiting if the 
generation count changes.

-- 
John Baldwin